The present invention is ideally suited for use with an artificial leg or prosthesis worn by an above knee amputee, but also has other applications and uses. Normally this type of prosthesis involves an artificial knee joint including a socket for receiving and engaging the stump of the user, a knee bracket rigidly connected to the socket, and a frame which extends downwardly from the bracket and is pivotally connected to the bracket by a horizontal shaft. A pylon and artificial foot are connected to the base of the frame, and a control unit is connected for locking the knee joint to prevent it from buckling under load in the stance phase of a step, and for freeing the knee joint in the swing phase of the step. Preferably, the prosthesis controls the knee joint in such a way that the amputee will walk with a normal or natural appearing gait. This gait is characterized by almost identical movements performed by both lower limbs at varying walking speeds.
The biological or natural knee joint is powered by the actions of muscles. Each muscle develops an active force by contraction and also provides variable stiffness or resistance. It has not been feasible to duplicate muscle contraction in leg prosthesis because of the weight and bulk that would be required to duplicate this function. Research has focused on implementing stiffness or resistance to rotation of the knee joint. Usually this involves switching the knee joint between one of two modes, locked or free to rotate. The locked mode occurs during the stance phase of the gait cycle, and the free to rotate mode occurs during the swing phase of the gait cycle. The stance phase applies when the foot of the prosthesis is on the ground, and the swing phase applies during the time when the foot of the prosthesis is off the ground.
Much of the research in recent years has sought improvements in controlling an artificial knee joint as a way to improve gait and enable the amputee to deal with situations such as descending stairs or ramps, or lowering into a sitting position. If a knee joint is considered a simple hinge, there are two separate actions which occur. During flexion, the upper and lower segments move closer together during rotation of the knee joint. During extension, the leg straightens and the segments move apart. For a prosthetic knee joint to duplicate a biological knee, it is necessary to control the resistance to rotation in each direction independently and variably. This resistance to rotation during swing phase can be accomplished with a mechanical damper or friction device, a pneumatic damper, or a hydraulic damper. It is generally accepted in prosthetics that a hydraulic damper provides the smoothest action over a wider range of walking speeds.
Stance phase control must provide a very high resistance to flexion or lock completely any rotation to flexion. Stance control is usually provided by a weight activated mechanical locking brake mechanism, or a position activated polycentric linkage system, or a position activated hydraulic damper. Mechanical braking mechanisms can be difficult to keep adjusted properly and can cause the amputee to walk with a slightly unnatural gait. Position activated polycentric mechanisms require more concentration and can be difficult for amputees to use in some situations. Hydraulic dampers, while providing a more natural gait, require more concentration and training for the amputee.
U.S. Pat. Nos. 5,405,409 and 5,443,521, which issued to the assignee of the present invention, disclose a linear type hydraulic damper for controlling an above knee prosthesis. This hydraulic damper has independently adjustable and variable resistance in flexion and extension during the swing phase of the gait cycle. Because of the turbulent flow of the hydraulic fluid during the swing phase, this damper can accommodate a wide variation of gait speeds. The control damper has a single damping rate in stance phase that can be manually adjusted for each amputee's need. When the knee joint is fully extended, the damper assumes a non-stance resistance mode. This position activated stance phase can initially require extra gait training and concentration on the part of the amputee to receive full benefit of the damper.
Electronics have recently been introduced into lower extremity prosthetics in an attempt to make walking easier for the amputee. For example, U.S. Pat. No. 5,062,856 and U.S. Pat. Nos. 5,383,939 and 5,571,205 disclose two systems which use a microprocessor control to adjust the resistance in a pneumatic or hydraulic cylinder during swing phase in an attempt to provide control of rotation of the knee joint over a wider range of walking speeds than is available with standard pneumatic or friction dampers.
Further improvement in amputee gaits could come from a mechanism that in the beginning of the stance phase would allow for a small amount of knee flexion and then lock against further flexion while simultaneously allowing for knee extension as the leg straightens due to body action. Such a mechanism is described by Siegmar et al in "Design Principles, Biomechanical Data and Clinical Experience with a Polycentric Knee Offering Controlled Stance Phase Knee Flexion: A Preliminary Report", Journal of Prosthetics and Orthotics, Vol. 9, No. 1, pp. 18-24, Winter 1997, and by Popvic et al in "Optimal Control for an Above-knee Prosthesis with Two Degrees of Freedom", J. Biomechanics, Vol. 28 No. 1, pp. 89-98, 1995.
An amputee needs different resistance to knee flexion during stair descent than is needed while sitting down in a chair. Accordingly, it is desirable for a control mechanism to be capable of providing these different resistances to knee flexion automatically. The control mechanism should also provide for swing resistance over a wide range of gait speeds. All of this should happen automatically so that the amputee can walk without having to think about his prosthesis.
The same type of computer controlled hydraulic damper system that can be used with amputees can also be used on other applications such as robotics, braking systems, and exercise equipment. These applications only vary in the size of the actuator to control the maximum resistance applied. They all may use common sensors, microprocessor controlled electronics, and valve technology. Computer controlled exercise equipment are disclosed in U.S. Pat. Nos. 4,354,676, 4,711,450, 4,919,418, 5,230,672, and 5,397,287. In any such equipment, it is desirable to be able to maintain accurate applied resistance over a wide range of temperature and manufacturing tolerances. It is also desirable to have proper feedback control and a hydraulic valve and controller designed for relatively slow speeds of operation.